We don't know exactly, probably Bran doesn't know exactly either and if there is anything extra that three eyed raven should do except knowing everything, it is yet to be revealed... and most likely surprise us. There is nothing mentiond in the show before that may give us a clue.
One point of "getting ready" is what Bran said himslef. He has the power to see everything but he still needs to work on his powers, everything is still fragmented and he needs to look more to see the whole picture. So in this aspect, he needs to see and understand more in hope he will learn more useful things for battle against the dead, maybe what Nigh King wants or what is his weak spot.
Second possible aspect is warging. We can go crazy with theories what he can achive with this, but I doubt his uncle has this specific ability in mind. Seeing everything seems more likely.
The only thing that is left is that unexpected "existing anywhere" ability and we can also make various speculations what will Bran be able to do after Hodor incident.
And the last and most likely is that Benjen didn't have anything specific in mind, he just knew Bran must work further on his powers and more he is capable of before the night comes the better.
If you suspect something more specific, only Martin and producers of the show can (but probably won't) answer. We just have to wait and see.